1. Field of the Invention
The present invention relates generally to a device and method for spreading channels in a CDMA communication system, and in particular to a channel communication device and method for a CDMA communication system having different spreading rates.
2. Description of the Related Art
In order to increase channel capacity, a CDMA (Code Division Multiple Access) communication system spreads channels using orthogonal codes. For example, a forward link of an IMT-2000 system performs channel spreading using orthogonal codes. An example of an orthogonal code that is typically used is a Walsh code.
The number of available orthogonal codes is determined depending upon a modulation method and a minimum data rate. However, in the proposed IMT-2000 CDMA system, the channels assigned to the users will increase in number in order to improve system performance. To this end, the future CDMA system includes a plurality of common channels and dedicated channels and assians the channels to the mobile stations, thereby increasing channel capacity.
However, even in the proposed IMT-2000 CDMA system, an increase in the utilization of the channels limits the number of available orthogonal codes. Further, the reduced number of available Walsh orthogonal codes limits the increase in channel capacity. In an effort to solve this problem, there is proposed a method for using quasi-orthogonal codes for channel spreading codes which have a minimum interference with the orthogonal codes and have a variable data rate.
In the IMT-2000 system, a 1xc3x97 system uses a spreading code group having a spreading code rate 1, and a 3xc3x97 system uses a spreading code group having a spreading code rate 3. In this case, the 1xc3x97 system has spreading codes with a maximum length of 128 as shown in FIG. 1A, and spreads code symbols by selecting a spreading code corresponding to a designated spread code index. Further, the 3xc3x97 system includes spreading codes with a maximum length of 256 as shown in FIG. 1B, and spreads the code symbols by selecting a spreading code corresponding to a designated spreading code index.
Referring now to FIG. 2, there is shown the channel characteristics at a frequency band of a multi-carrier mobile communication system. The 1xc3x97 system uses a single carrier whose channel band is 1.25 MHz, and the 3xc3x97 system uses 3 carriers, each distributed to a different channel band and having a combined channel band of 3.75 MHz. A unique orthogonal code is assigned to each carrier. When the 1xc3x97 system is overlaid with the 3xc3x97 system as shown in FIG. 2. It is not possible to guarantee a correlation property between a spreading code of length 128 using a mask function at a spreading rate 1 and a spreading code of length 128 using a mask function at a spreading rate 3, at each 1.25 MHz band. Therefore, interference may increase between a user using the masking function at the spreading rate 1xc3x97 and another user using the mask function at the spreading rate 3xc3x97.
A problem concerning the correlation property will be described with reference to FIG. 3. FIG. 3 is a diagram for explaining the mutual interference between a user of the 1xc3x97 system using a quasi-orthogonal code and a user of the 3xc3x97 system using an orthogonal code.
In FIG. 3, T1 denotes the number of chips into which the quasi-orthogonal code user of the 1xc3x97 system spreads one symbol and T2 denotes the number of chips into which the orthogonal code user of the 3xc3x97 system spreads one symbol. Interference that the quasi-orthogonal code (Qm+Wk) user of the 1xc3x97 system, experiences from the orthogonal code (Wj) user of the 3xc3x97 system can be given by the equation:                                                                         ∑                i                                  T                  i                                            ⁢                              (                                                      Q                                          m                      ,                      i                                                        +                                      W                                          k                      ,                      i                                                                      )                                      +                          W                              j                ,                i                                              =                                                                      ∑                  i                                      T                    i                                                  ⁢                                  Q                                      m                    ,                    i                                                              +                              (                                                      W                                          k                      ,                      i                                                        +                                      W                                          j                      ,                      i                                                                      )                                      =                                                                                ∑                    i                                          T                      i                                                        ⁢                                      Q                                          m                      ,                      i                                                                      +                                  W                                      s                    ,                    i                                                               less than                               Θ                min                                                    ⁢                  xe2x80x83                                    (        1        )            
That is, the interference satisfies an upper limit formula of the correlation for the quasi-orthogonal code. Therefore, in this case, there is no serious matter.
FIG. 4 is a digram for explaining mutual interference between channels each using quasi-orthogonal codes of different lengths. In FIG. 4, 1xc3x97 denotes a user using a quasi-orthogonal code of length 128 and 3xc3x97 denotes a user using a quasi-orthogonal code of length 256. In this case, interference that the quasi-orthogonal code (Qm+Wk) user of the 1xc3x97 system experiences from the quasi-orthogonal code (Qn+Wj) user of the 3xc3x97 system does not satisfy the upper limit formula, as shown in Equation (2) below:                                                         ∑              i                              T                i                                      ⁢                          (                                                Q                                      m                    ,                    i                                                  +                                  W                                      k                    ,                    i                                                              )                                +                      (                                          Q                                  n                  ,                  i                                            +                              W                                  j                  ,                  i                                                      )                          =                                                            ∑                i                                  T                  i                                            ⁢                              (                                                      Q                                          m                      ,                      i                                                        +                                      Q                                          n                      ,                      i                                                                      )                                      +                          (                                                W                                      k                    ,                    i                                                  +                                  W                                      s                    ,                    i                                                              )                                =                                                    ∑                i                                  T                  i                                            ⁢                              (                                                      Q                                          m                      ,                      i                                                        +                                      Q                                          n                      ,                      i                                                                      )                                      +                          W                              s                ,                i                                                                        (        2        )            
In this case, the mutual interference between the channels increases.
Therefore, when using the quasi-orthogonal codes of spreading code groups having different lengths, the mobile communication system stores the spreading codes of different lengths, and thus increases the hardware complexity. Further, using the spreading codes having different spreading rates deteriorates the interference property between two users thereby causing performance degradation.
It is, therefore, an object of the present invention to provide a communication device and method for a CDMA communication system wherein a base station device spreads and despreads channel signals using spreading codes having the same length as those of mobile stations having different spreading rates.
It is another object of the present invention to provide a communication device and method for a CDMA communication system wherein a mobile station spreads and despreads channel signals using spreading codes having the same length as those of base stations having different spreading rates.
To achieve the above objects, there is provided a channel spreading device for a base station device in a CDMA communication system for spreading input symbols using a first quasi-orthogonal code having a first length or channel spreading the input symbols using a second quasi-orthogonal code having a second length which is a multiple of the first length. The device comprises a spreading code generator for generating the second quasi-orthogonal code; a channel spreader for providing a plurality of durations by dividing a duration of the second quasi-orthogonal code by the multiple of the length of the first quasi-orthogonal code, and spreading channel coded symbols with the quasi-orthogonal codes having the plurality of durations described above; and a PN spreader for spreading the channel spread signal with a PN code.
The spreading code generator comprises a controller for generating a mask index and a Walsh code index corresponding to a received index for generating the second quasi-orthogonal code; a mask generator for generating a mask of the second quasi-orthogonal code corresponding to the mask index; a Walsh code generator for generating a Walsh code corresponding to the Walsh code index; and a circuit for mixing the mask of the second quasi-orthogonal code with the Walsh code to generate the second quasi-orthogonal code to use as the spreading code.
When the length of the second quasi-orthogonal code is 2 times the length of the first quasi-orthogonal code, the channel spreader spreads, at a spreading rate of the first quasi-orthogonal code, one symbol with a first half-period chip signal of the second quasi-orthogonal code output from the spreading code generator and another symbol with a second half-period chip signal of the second quasi-orthogonal code, whereby two symbols are spread at a duration of one spreading code.